Essay on Air Pollution: Sources, Causes, Effects and Control

Air pollution occurs due to the presence of undesirable solid or gaseous particles in the air in quantities that are harmful to human health and environment.

It can be defined as presence of foreign matter-either gaseous or particulate or combination of both in the air which is detrimental to the health and welfare of human beings.

Sources of Air Pollution:

1. Primary Pollutants:

Pollutants that are emitted directly from identifiable sources are produced by natural events can be in the form of particulate matter or gaseous form. These are called primary pollutants Example: Dust storms and volcanic eruptions and through human activities like emission from vehicles, industries etc.

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There are five primary pollutants that contribute to 90% of global air pollution:

(i) Oxides of carbon like carbon monoxide and carbon dioxide (CO and CO2).

(ii) Oxides of nitrogen, like NO, NO2, NO3 (expressed as NOX).

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(iii) Oxides of sulphur particularly sulphur dioxide (SO2).

(iv) Volatile organic compounds, mostly hydrocarbons.

(v) Suspended particulate matter (SPM).

Particulate pollutants are categorized according to size, mode of formation (source) or physical state.

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a. Aerosol (general term for particles suspended in air)

b. Mist (aerosol containing liquid droplets e.g., H2SO4 mist)

c. Dust (aerosol containing solid particles)

d. Smoke (aerosol containing mixture of solid and liquid particles produced by chemical reaction such as fire)

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e. Fume (same as smoke by produced by condensation of hot vapors of metals e.g.. Zinc and Lead fumes)

f. Plume (smoke coming out of chimney)

g. Fly ash (finely divided non-combustible particles present in the gases arising from fuel combustion. It contains inorganic metallic or mineral substances released when the organic part of the coal is burnt.)

Causes of Air Pollution:

Air pollution may originate from one or more variety of sources. The natural pollution includes sources such as oceanic aerosol, volcanic emissions, biogenic sources, windblown terrestrial dust and lightening. The artificial pollution generates from human activities and includes sources such as fuel burning, refuge burning, transportation, construction of buildings, chemical factories, metallurgical factories and, vehicles. The third category includes solvent usage and sources include spray painting and solvent extraction. Automobiles are the first rate of polluters. Industries occupy second position.

Exposure to air containing even 0.001% of CO for several hours can cause collapse, coma and even death. As CO remains attached to hemoglobin in the blood for a long time, it accumulates and reduces the oxygen carrying capacity of blood. This impairs thinking, causes headaches, drowsiness and nausea. SO2 irritates the respiratory tissues.

NO2 can irritate lungs; aggravate asthma and susceptibility to influenza and common colds. Many volatile organic compounds (benzene and formaldehyde) and toxic particulates can cause mutations and cancer. Lead causes neurological problems and cancer.

(ii) Effects on plants:

Gaseous pollutants enter the leaf pores and damage the leaves of crop plants, interfere with photosynthesis and plants growth and reduces nutrient uptake and causes the leaves to turn yellow, brown or drop off altogether.

(iii) On materials:

Air pollutants break down the exterior paint on cars and houses.

(iv) Ozone Layer Depletion:

The upper stratosphere consists of considerable amounts of ozone, which works as an effective screen for UV light. This region is called ozone layer, which extends up to 60 km above the surface of the earth. Ozone is a form of oxygen with 3 atoms instead of 2.

It is produced naturally in the atmosphere. Presence of certain pollutants can accelerate the breakdown of ozone. Depletion of ozone effects human health, food productivity and climate. Sun burn, cataract, aging of skin and skin cancer are caused by increased UV radiation.

It weakens the immune system by supporting the body’s resistance to certain infections like measles, chickenpox and other viral diseases. UV radiation affects the ability of plants to capture light energy during the process of photosynthesis. This reduces the nutrient content and growth of plants mostly in legumes and cabbage.

Plants and animals are damaged by UV radiations. Contribute to global warming, a phenomenon which is caused due to the increase in concentration of certain gases like CO2 , NO2 methane and chloroflorocarbons (CFCs).

(v) Photochemical Smog:

Photochemical smog is highly oxidising polluted atmosphere comprising largely of ozone, NOX, hydrogen peroxide, organic peroxides, PAN and peroxybenzoil nitrate (PBzN). This is produced as a result of photochemical reaction among NOx hydrocarbons and oxygen.

Effects of photochemical smog are as follows:

a. Breathing ozone results in respiratory distress, headaches.

b. PAN damages plants.

c. PAH’s poly nuclear aromatic hydrocarbons, carcinogenic.

(vi) Acid Rain:

Acid rain is a rain or any other form of precipitation that is unusually acidic, i.e., elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure through the process of wet deposition. Acid rain is caused by emissions of compounds of ammonium, carbon, nitrogen, and sulfur which react with the water molecules in the atmosphere to produce acids.

H2O (1) + CO2 (g) → H2CO3 (aq)

Carbonic acid then can ionize in water forming low concentrations of hydronium and carbonate ions.

2H2O (1) + H2CO3 (aq) → CO32- (aq) + 2H3O+ (aq)

Acid deposition as an environmental issue would include additional acids to H2CO3.

The effects of acid rain have been known for a long time. Though experts admit that the problem of acid rain is generally under control, it’s still worth reiterating the effects brought on by acid rain on the wider environment.

Acid rain has adverse effects on:

a. Forests and other vegetation

b. Freshwater lakes and streams destroying aquatic life

c. Soil

d. Buildings and materials

(vii) Global Warming:

Carbon dioxide and other gases allow light to pass, but trap heat in the atmosphere much like glass in a greenhouse traps heat. This greenhouse effect is thought to be responsible for global warming. Carbon dioxide contributes to only 56% of greenhouse heating. The average surface temperature of Earth is about 15°C (59°F). Global Warming is increase in the average temperature of the atmosphere, oceans, and landmasses of Earth.

Effects of global warming are as follows:

a. Temperature extremes

b. Rise in sea level, and change in precipitation

c. Injuries from storms, coastal flooding

d. Interruption of power supply, contamination of drinking water

e. Droughts

f. Food shortages due to shift in agricultural food production

g. Air pollution (made worse by warming)

h. Asthma, bronchitis, emphysema complications

i. Strain on public health systems

j. Increased need due to population migrations

k. Unable to contain spread of infectious diseases

Taj Mahal, Agra, India: A symbol of love affected by air pollutants:

Emperor Shah Jahan constructed this historic monument in the memory of his wife Mumtaz Mahal at Agra, India in the year 1648. The innumerable industries in and around Agra released a large amount of sulphur dioxides, suspended particulate matter, smoke, soot etc causing tanning, blackening, and yellowing of the marble stone.

The oxides reacting with rainwater resulted in acid precipitation and in return caused the deterioration and corrosion of the Taj Mahal. In 1984, the Supreme Court of India ordered designated polluting industries in the Taj area to be shut down and or adopt cleaner control technologies.

Control of Air Pollution:

Zoning:

The zoning of the industries is done based on the type of industries, their function etc. every city has its own zoning rules.

Zoning of the industries may be based on:

(a) Their Functions:

The functions include industries such as linkages for industries, sidings etc.

(b) Performance:

The performance includes classification of industries according to their nuisance value such as:

If zoning is done properly, it results inconsiderable improvement of health of the community as a whole. It prevents the invasion of undesirable in and around residential areas and so toxic, hazardous and harmful gases and odours are prevented from entering or attacking the humans living in residential areas.

Air Pollution Control at Source:

The air pollution problem can be minimized at the source of making use of the following measures:

(a) Raw Material Substitution:

In order to check air pollution, it is desirable to substitute the raw material if it results in pollution, by another one which is less polluting. The raw material may contain an ingredient which is not essential but a pollution source. The pollution can be minimised if the non-essential ingredient is removed before the processing of the raw material. Low sulphur fuels can be used in place of high sulphur fuels in order to control air pollution by SO2.

(b) Modification of Process:

Pollution reduction can frequently be achieved by using modified procedure or new process. For example, the use of exhaust hoods and ducts over several types of industrial ovens allow the recovery of various solvents that could have become air pollutants.

(c) Equipment Alterations:

Equipment alterations such as the use of floating roof tanks rather than vented tanks can cut down on the evaporation losses. New type of equipment for example, the basic oxygen furnaces which are replacing the open hearth furnaces in steel industry, pose much less air pollution problems.

Controlling Air Pollution by Devices:

Because of large number of industries, various types of gases are liberated in the atmosphere along with particulates and become major source of air pollution. In order to prevent these pollutants into the atmosphere, control devices have been used depending on the collection property and capacity of the device and nature of the processes used by the particular industry. Based on the method of removal several types of dust collectors have been used. Some important control devices are gravity settling chamber, fabric filters, wet scrubbers and electrostatic precipitator.

Following are the 5 classes of the particulate collection equipment:

(i) Gravity Settling Chamber:

This is the simplest method for the control of particulate matter. This applies to coarse particulate matter larger than 50 µm. Here the gravitational force is employed to remove particulate matter with a settling velocity greater than 0.13 m/s. they are used to control emission of coarse particulate from power plants and industrial sites.

(ii) Cyclone Separator:

Cyclone separator is gas cleaning devices that employ a centrifugal force generated by a spinning gas stream to separate the particulate matter from the carrier gas. The cyclone separator is usually employed for removing particles 10 µm in size and larger.

Fractional removal efficiency of the cyclone separator drops rapidly beyond a certain particle size. Cyclone separator operates by the centrifugal force which is several times greater than the gravitational force.

(iii) Wet Collectors:

In a wet collector a liquid, usually water is used to capture particulate matter or to increase the size of aerosols. It is used for fine particulates, ranging from 0.1 to 20 µm. One of the primary aims of the device is the adequate dispersion of the liquid phase in order to achieve good contact between the particulate phase and the liquid phase.

Three major types of wet collectors are:

(a) Spray chamber scrubbers

(b) Cyclonic scrubbers

(c) Venturi scrubbers

(iv) Fabric Filters:

Filtration is one of the oldest and most widely used methods of separating particulate from the carrier gas. A filter generally any porous structure is composed of granular or fibrous material which tends to retain the particulate as the carrier gas passes through the void of the filter.

Fabric filters are usually formed from cylindrical tubes and hung in multiple rows to provide large surface areas for gas passage. Typical dust loadings handled are from 0.23 to 23 gm/m3 of gas. Fabric filters have efficiencies of 99% or better when collecting 0.5 µm particles and can remove substantial quantities of 0.01 µm particles.

(v) Electrostatic Precipitator:

Particulate and aerosol collection by electrostatic precipitation is based on the mutual attraction between particles of one electric charge and the collecting electrode of opposite polarity.